EMC VNX Direct Driver Consistency Group support

Consistency Group support is newly introduced in Juno.

This commit is to add the changes in EMC VNX Direct Driver to
support CG.

Implements: blueprint emc-vnx-direct-driver-cg-support

Change-Id: I6c30ef2609ca8a559f5635129ce6a7c960a9f0a7
Merge "EMC VNX Direct Driver Consistency Group support"